The present invention relates to an apparatus for internal combustion engines that variably controls valve timing and valve lift.
Devices that vary the valve lift or valve timing of intake valves and exhaust valves in automobile engines are known in the prior art. For example, Japaneses Utility Model Publication No. 3-4730 describes an apparatus that drives a rocker arm using a high speed cam and a low speed cam.
The rocker arm has two arm portions. The first arm portion is provided with a shifting mechanism that selectively locks or unlocks the first arm portion with respect to a plunger. The plunger follows a high speed cam. The second arm portion follows the low speed cam. The low speed cam has a lift portion (cam nose), the radius of which is smaller than the radius of the lift portion of the high speed cam. When the plunger is locked to the first arm portion by the shifting mechanism, the low speed cam and the second arm portion are separated from each other. Thus, the motion of the low speed cam is not transferred to the second arm portion. The motion of the high speed cam moves the plunger and the second arm portion, which is integrally locked to the plunger. This drives the rocker arm and opens or closes a valve.
When the shifting mechanism unlocks the plunger and permits the plunger to move freely, the motion of the high speed cam is transferred to the plunger but not the rocker arm. Hence, the low speed cam drives the rocker arm and opens or closes the valve.
A typical shifting mechanism will now be described with reference to FIG. 12. As shown in the drawing, a rocker arm 1 is fixed to a rocker shaft 2. The rocker arm 1 includes a rod 3, which is driven by hydraulic pressure. A plate-like lock member 4 is secured to the distal end of the rod 3. A slit 4a bifurcates the distal end of the lock member 4. The rocker arm 1 is further provided with a cylindrical guide 7. A plunger 5 is inserted into the guide 7 and is supported so that it is axially movable. The lock member 4 can be moved into the path of the plunger 5. The guide 7 is provided with an elongated hole 7a to receive the lock member 4. The lock member 4 is received by the elongated hole 7a when extended into the path of the plunger 5.
When the rod 3 is projected by hydraulic pressure, the lock member 4 moves above the plunger 5 and engages the top surface of the plunger 5. Thus, the upward movement of the plunger 5 is restricted and the plunger 5 is held below the lock member 4. As a result, the plunger 5 and the rocker arm 1 are rocked integrally with each other by a high speed cam (not shown). This causes the rocker arm 1 to lower a poppet valve 6.
When hydraulic pressure is not applied to the rod 3, the rod 3 is moved toward the left from the position shown in FIG. 12. As a result, the lock member 4 moves out of the path of the plunger 5. This permits the plunger 5 to move axially with respect to the locker arm 1. In this state, the plunger 5 is moved axially by the motion of the high speed cam while the rocker arm 1 is driven by a low speed cam (not shown).
The lock member 4 that locks the plunger 5 is a flat plate. To receive the lock member 4 when the lock member 4 extends into the path of the plunger 5, the guide 7 must be provided with the elongated hole 7a. However, the machining of the elongated hole 7a in the guide 4 is burdensome. Furthermore, it is difficult to accommodate the shifting mechanism entirely in the rocker arm 1.
The lock member 4 is moved into the path of the plunger 5 to lock the plunger 5 to the rocker arm 1. In this state, the force of the plunger 5 is transmitted to the rocker arm 1 through the lock member 4. In other words, the plunger 5 applies a shearing force to the lock member 4. Therefore, the lock member 4 must have sufficient strength to withstand this force. As a result, the thickness of the lock member 4 must be increased. This enlarges the size and increases the weight of the shifting mechanism.